Examples of typical molding methods of ceramics include various methods such as extrusion molding, injection molding, and slip casting. With respect to the extrusion molding, for example, this is a method configured to conduct molding by applying a pressure to a kneaded composition (hereinafter referred to as “green ceramic clay”) obtained by kneading a ceramic powder, a binder, water, and the like to thereby extrude a green ceramic clay through a die, and is thus suitable for producing rod-like or pipe-like products each having a constant cross-sectional shape, with a good efficiency. Although the extrusion molding is also used to produce filters, catalyst carriers, and the like, it has been strongly desired in recent years to achieve highly precise extrusion molding so as to improve performances of such products.
To obtain a highly precise extrusion molded article, it is required to improve moldability of a green ceramic clay upon extrusion molding. Patent Literature 1 discloses a method configured to add a dispersant comprising a specific fatty acid salt to thereby lower a viscosity of a green ceramic clay. This method is problematic in that the moldability is still insufficient, and in that smooth molded bodies are hardly obtained. Further, Patent Literature 2 discloses a dispersant containing a polyoxyalkylene unit excellent in lubricating ability of a molded article. It is required to use a large amount of dispersant in order to sufficiently lower a viscosity of a green ceramic clay, such that the dispersant obstructs an interaction between binder molecules, thereby occasionally and considerably deteriorating a shape-retaining ability, a molded article strength, and the like.
In turn, the present applicant has aimed at simultaneously providing a green ceramic clay with a higher flowability upon extrusion molding and ensuring a shape-retaining ability of a molded article after extrusion, and thus has proposed a production method of a porous ceramic, configured to adopt specific polymer microparticle having a water absorbing ability in Patent Literature 3. In order to simultaneously achieve a higher flowability upon extrusion molding and ensure a shape-retaining ability after extrusion by adopting such polymer microparticle, it is required to attain such a design that: part of water is released from the polymer microparticle by a pressure applied upon extrusion molding, and then the polymer microparticle quickly absorb the water therearound after the pressure is released to thereby enhance a hardness of a molded article.
Nonetheless, even in case of adopting the polymer microparticle according to the disclosure of Patent Literature 3, ionic impurities are released when residual monomers therefor are present in a large amount, such that the impurities obstruct the water absorbing ability of the polymer microparticle in a manner to occasionally lead to an insufficient water absorbing ability of the polymer microparticle in a green ceramic clay, thereby resultingly and occasionally failing to satisfactorily achieve simultaneous establishment of a higher flowability of the green ceramic clay upon extrusion molding and an assurance of a shape-retaining ability of a molded article after extrusion.